The substituted pyrroles of this invention are useful as intermediates in the syntheses of polymer additives which in turn are effective to protect various polymers from deterioration caused by exposure to ultraviolet light. The substituted pyrroles of the invention can be reacted with acetylene dicarboxylic acid, in a Diels-Alder reaction, for example, to form a much larger, less volatile compound which is effective to inhibit the deterioration of polypropylene upon exposure to ultraviolet light. This Diels-Alder product, containing an olefinic double bond, may be hydrogenated to yield a product which is an even more effective ultraviolet light stabilizer. Other active dienophiles may of course also be used for this purpose and illustrative examples include dialkyl maleates, alkyl acrylates and methacrylates, diethyl acetylene dicarboxylate, propargyl alcohol and butynediol.
The effectiveness of these Diels-Alder reaction products as light stabilizers is believed to be due to the hindered amide group.
Also, the acyl and phosphoryl groups may be removed (by hydrolysis) from these Diels-Alder condensation products and the resulting hindered amines likewise are effective ultraviolet light stabilizers in polymer compositions.
While a wide variety of polymers are benefitted by the protective action of these Diels-Alder products, olefin polymers are especially benefitted. Polypropylene, in particular, is susceptible to stabilization by the addition of a small proportion of such an additive.
The condensation of gamma-diketones such as hexane-2,5-dione, i.e., acetonylacetone, with primary amines to form pyrroles, is shown at page 77 of "The Chemistry of Pyrroles" by Jones et al., Academic Press (1977). The reaction is referred to as the Paal-Knorr condensation. It appears that the condensation reactions were carried out in aqueous systems because there is a considerable discussion about the optimum pH at which the reaction may be carried out. Moreover, it is stated that 2,5-dimethylpyrrole may be prepared from the reaction of hexane-2,5-dione and formamide; such a result was obtained in an aqueous environment.